The present invention relates to a high-density mounting method and structure for an electronic circuit board and, more particularly, to a high-density mounting method and structure for an electronic circuit board suitably used in mounting a semiconductor chip, a surface mounted device (SMD), a multichip module (MCM), and the like on a printed circuit board with a high density.
Conventionally, as a high-density mounting method and structure for an electronic circuit board of this type, the following ones are known. Japanese Patent Laid-Open No. 1-175297 discloses a multilayer printed circuit board device in which semiconductor chips are buried in a board in order to perform high-density mounting. More specifically, in order to solve the problems posed by dipping the printed circuit board in a developing solution that adversely affects the semiconductor chips and the warp caused by mounting, two boards in which through holes having the sizes of the semiconductor chips are formed at the same positions are stuck on the two surfaces of one base board, thereby constituting a wiring board.
Japanese Patent Laid-Open No. 57-7147 discloses the mounting construction of a semiconductor device in which LSI chips are buried in a substrate to achieve high-density mounting. More specifically, a ball bump is formed on each LSI chip by bonding, and the LSI is buried in a recessed portion formed in an insulating substrate and is sealed. Subsequently, the sealed surface is flatly ground to expose the ball bump portion, and is metallized. Since the ball bump can be conducted and connected to a circuit by metallization, if the metal portion is removed to leave a necessary portion, a wiring pattern is formed.
The conventional high-density mounting method and structure for the electronic circuit board described above have the following problems. In the former case, semiconductor chips are buried in the upper and lower surfaces of the board to eliminate the warp. In practice, however, it is considerably difficult to arrange the semiconductor chips on the upper and lower surfaces of the board in terms of mounting the chips on the board and formation of a circuit, and the mounting operation is largely limited. Therefore, this device can be used in limited applications.
Boards in which holes are formed are stacked on the upper and lower surfaces of the base board for the purpose of formation of recessed portions, thereby realizing a multilayer structure. If the boards are pressed through a prepreg so that they are adhered to each other, the prepreg is squeezed to the peripheries of the boards and flows into the recessed portions as well. In other words, it is difficult to form precise recessed portions in the board. In order to bury the semiconductor chips, the squeezed prepreg must be removed afterwards.
In order to perform circuit formation without dipping the semiconductor chips in the developing solution, a wiring pattern is formed by printing a conductive paste. When recessed portions are formed in the board and the semiconductor chips are fitted in the recessed portions, gaps are formed inevitably. Accordingly, if a circuit is formed by printing on the board in which the semiconductor chips are fitted, the circuit forms a bridge even partially and tends to cause disconnection. In order to avoid this, if the circuit is formed by plating, this is against the original purpose of not dipping the printed circuit board in the plating solution. Even if the circuit is formed by plating, masking must be performed after plating, making the manufacturing method complicated.
In the latter case, when forming a wiring pattern to be connected to an LSI chip, a circuit is formed by etching after metallization. Etching generally employed in a printed circuit board is performed in the following manner. A photosensitive dry film is brought into tight contact with the printed circuit board. Only a necessary portion of the photosensitive dry film is photosensitized, and a portion of the photosensitive dry film other than the circuit portion is removed by etching. With this method, however, a flat place where the dry film can be adhered is required, and the step of flatly grinding the dry film until the bump of the LSI chip is exposed is required.
When connecting the bump and the circuit by metallizing the exposed bump, since the thermal expansion coefficient of the sealing member and that of the bump differ, if the size of the bump decreases, the connection reliability suffers when influenced by the thermal stress. In particular, the recent IC pad pitch is 100 .mu.m or less. If the IC pad pitch becomes small in this manner, the connection area becomes very small, posing a significant problem in reliability.
Basically, since the board is not a multilayer board but a double-sided board, connection of a complicated circuit is limited. When a large number of recessed portions are formed, a warp is caused in the entire board due to the shrinkage of the sealing resin.